The effects of inoculum and medium composition (i.e. potato dextrose broth as carbon source, soybean peptone, calcium carbonate, and metal ions) on pellet formation of Rhizopus oryzae ATCC 20344 have been studied. Metal ions were found to have a significant negative effect on pellet formation while soybean peptone had a positive effect. In addition, potato dextrose broth and calcium carbonate were beneficial to R. oryzae for growing small, smooth pellets during the culture. The study also demonstrated that an inoculum size of less than 1.5 x 10(9) spores/L had no significant influence on pellet formation although it had large impacts on pellet growth. Thus, a new approach to form pellets has been developed using only potato dextrose broth, soybean peptone, and calcium carbonate allowing for pellet size to be controlled by adjusting inoculum size and the concentrations of potato dextrose broth, soybean peptone, and calcium carbonate in the medium. Published by Elsevier Ltd.